Conventionally, an illumination system in which a power source, an illumination load appliance and a controller are connected in series and the controller performs illumination control of the illumination load appliance is sometimes employed. In such illumination system, power is supplied to the illumination load appliance using two-wire wiring. The controller adjusts the power supplied to the illumination load appliance by means of a phase control method to perform dimming control (for example, Japanese Patent Application Laid-Open Publication Nos. 2007-538378 and 2005-011739).
In such two-wire wiring illumination system, e.g., a bidirectional triode thyristor (hereinafter, referred to as “TRIAC”) is used as a switching element configured to perform power phase control. By turning on/off the TRIAC, the power supply from the power source to the illumination load is controlled, whereby dimming is performed. In other words, the TRIAC is turned on a period of delay time, which is based on the dimming control, from a zero crossing of the power source voltage, whereby the time of supplying power to the illumination load is controlled to perform dimming.
In such power phase control method, since the power is steeply turned on, power supply noise to be generated is large. In order to reduce the effect of such power supply noise, a noise prevention circuit including a capacitor and an inductor is employed. A dimmer including such noise prevention circuit is disclosed in, e.g., Japanese Patent Application Laid-Open No. 11-87072.
However, a resonant circuit is formed by the capacitor and the inductor included in the noise prevention circuit, and when a TRIAC, which is a switching element, is turned on, the resonant circuit causes a resonant current to flow in the TRIAC. In other words, at the time of power supply using phase control, a transient oscillation occurs, and a resonant current (transient oscillation current) having a large peak value, which flows at that time, flows also into the TRIAC. It is necessary that a relatively large holding current flow in the TRIAC to maintain conduction. No problem arises during a period in which the resonant current flows in the TRIAC in the same direction as that of the current from the power source. However, during a period in which the resonant current flows in the opposite direction, the current flowing in the TRIAC may be relatively lowered to fall below the holding current.
Even in such case, where a bulb, which has a relatively low resistance value, is employed for the illumination load, the bulb, which is the illumination load, acts as a damping resistance, whereby the resonant current is suppressed, enabling a current equal to or higher than the holding current to flow in the TRIAC.
However, where a high-resistance element, such as an LED (Light Emitting Diode), is employed for the illumination load, immediately after the TRIAC is turned on, the current flowing in the TRIAC may be reduced by the resonant current to fall below the holding current, which causes the TRIAC to be turned off. Subsequently, the TRIAC may be turned on again. In this manner, the TRIAC may be repeatedly turned on/off in a half cycle of the power source voltage according to the level and polarity of the resonant current of the time when the TRIAC is on.
In other words, there has been a problem that depending on the type of the illumination load, the TRIAC may repeatedly be turned on/off even during a period in which the TRIAC is on under normal conditions, which causes flicker in the lighting.